1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire for use in truck and bus, more particularly it relates to a heavy duty pneumatic radial tire in which a belt is comprised of three cord layers for attaining weight reduction and cord breakage in an outermost cord layer is particularly controlled to improve a durability while maintaining and improving all tire performances such as cornering property and the like.
2. Description of Related Art
In this type of the heavy duty pneumatic radial tire for use in truck and bus, as shown in FIG. 8, a belt 20 is usually comprised of four cord layers 21-24, in which a first cord layer 21 located nearest to a carcass is arranged so as to incline cords at a relatively large cord angle with respect to a plane including a circumference of the tire (or an equatorial plane). Second and third cord layers 22, 23 are arranged to form a cross belt layer 25 by crossing cords of these layers with each other with respect to the plane including the tire circumference. A fourth cord layer 24 is arranged to incline cords in the same direction of extending the cords of the third cord layer and at a cord angle substantially equal to that of the third cord layer 23.
In the tire having such a belt 20, there is a possibility that when the tire rides on a sharp corner of rock or stone during running on a rough road of an uneven surface state scattering rocks or stones thereon, a cut failure arriving at the belt is caused. In the conventional tire, it is frequently attempted to avoid the damage due to such a cut failure (i.e. belt cord breaking-up) mainly by using an outermost cord layer or a fourth cord layer as a protection layer.
On the other hand, it has been proposed to decrease the number of the cord layers in the belt to three cord layers in the heavy duty pneumatic radial tire for attaining the weight reduction likewise passenger car tire or the like. In this case, the belt as usual is constructed by arranging cords of a first cord layer located nearest to the radial carcass at a relatively large cord angle with respect to the plane including the circumference of the tire and arranging second and third cord layers as a cross belt layer in which cords of these layers are crossed with each other with respect to the plane including the circumference of the tire and inclined at a relatively small cord angle with respect to such a plane. Such a tire is disclosed in, for example, JP-A-7-186613.
In the tire of JP-A-7-186613, there is adopted such a structure that when the belt is comprised of three breakers (corresponding to the cord layer), a tenacity per unit length of a third breaker viewed from the carcass is increased as compared with those of the remaining breakers because the tenacity of the third breaker is found out to be most lacking. This publication discloses that the above structure can cheaply and effectively prevent the occurrence of burst due to the breakage of the breaker when the tire rides on rock or stone.
As a result of the inventors' examination on the tire having such a structure, however, it has been confirmed that since the cords in the second and third breakers constituting the cross belt layer are crossed with each other at a relatively small cord angle, when a given internal pressure is applied to the tire in use, a large tension against such an internal pressure is applied to the cords in the second and third breakers and hence even if the tenacity (concretely strength at break) of the cord in the third breaker is increased at a state of applying the large tension to the third breaker, it is difficult to sufficiently control the breakage of the cord in the third breaker.
That is, when the cut failure is caused in the cord at a state of applying the above large tension, the cord is easily broken in proportion to the tension.
Furthermore, when the tire rides on a projection 31 such as a large stone or the like existing on a road surface 30 as shown in FIG. 3, bending force is applied to the belt in a direction of an arrow 32 and hence compression force is applied to the outermost cord layer. In this case, if a modulus of elasticity of a coating rubber for the outermost cord layer 28 is small, local buckling is apt to be created in cords of the outermost cord layer 28, so that there is a fear of causing cord breaking-up when the above buckling is repeatedly applied to the cords.